Catalytic converters have been used on gasoline-fueled automobiles produced in the United States since the mid-1970s for the purpose of promoting the oxidation of unburned hydrocarbons and of carbon monoxide. A few years after their introduction, the converters were adapted to promote the chemical reduction of oxides of nitrogen. At the present time, these converters typically employ small amounts of platinum and rhodium that are dispersed over a high surface area particulate carrier which, in turn, is distributed as a thin, porous coating (sometimes called a washcoat) on the wall of a ceramic monolith substrate or on the surface of a corrugated, oxidation-resistant stainless steel foil. The ceramic monolith is formed by an extrusion process providing hundreds of thin wall, longitudinal, parallel open cells per square inch of cross section, and the corrugated metal is wrapped or folded into a like configuration. These flow-through catalytic devices are housed in a suitable stainless steel container and placed in the exhaust stream under the vehicle downstream from the engine's exhaust manifold.
Under current practice, the converters are called three-way catalysts because they simultaneously effect the oxidation of carbon monoxide and unburned hydrocarbons and the reduction of oxides of nitrogen. This remarkable function is accomplished by controlling the proportions of air and fuel entering the engine such that the air-fuel ratio is continuously cycled closely about the stoichiometric air-fuel ratio. In this way, the exhaust gas passing over the catalytic surfaces of the converter is alternatively rich in oxygen and deficient in oxygen so as to promote the nearly simultaneous oxidation and reduction reactions. In some catalysts, the high surface area support material for the noble metal contains mixtures of high surface area ceria (CeO.sub.2) with high surface area forms of alumina. The ceria has been demonstrated to have the capability of temporarily storing oxygen during the oxygen-rich portions of the air-fuel cycles so as to provide oxygen for the oxidation reactions when the exhaust gas is momentarily oxygen deficient.
The careful control of the engine air-fuel ratio is enabled by the use of an on-board microcomputer which monitors and controls many functions of the engine in response to driver throttle manipulations. Such control of the air-fuel ratio is also abetted by the use of an oxygen sensor in the engine exhaust stream upstream of the catalytic converter. The oxygen sensor is an electrochemical device that uses a stabilized zirconia oxygen ion conductive electrolyte with a catalyzed electrode on one surface in contact with the exhaust gas and a reference electrode on an opposite surface. This oxygen sensor produces a voltage signal that signals the engine control computer as to whether the exhaust gas is then oxygen-rich or oxygen-lean. By use of the engine control computer, the oxygen sensor and other devices to closely monitor the air supply and fuel delivery to the cylinders of the engine, the air-fuel ratio can be adjusted every second or so.
Since the function of the engine and control of its exhaust is now largely dependent upon the engine control computer module and sensors that provide it with data, electronic diagnostic systems have been adapted to assure that these systems are functioning properly. In the event that some aspect of this closely controlled system is not functioning properly, a light is illuminated in the instrument panel to inform the driver and a fault code is registered in the computer to assist service technicians. See, for example, SAE Paper 911215, "On-Board Diagnostics for Emission Control Systems", Joseph M. Kotzan.
There will soon be further requirements for an on-board diagnostic system that is capable of monitoring all components or systems for which a malfunction can impact emissions, including the oxidation performance of the catalytic converter. See also, for example, SAE Paper 910561, "Detection of Catalyst Failure On-Vehicle Using the Dual Oxygen Sensor Method", John W. Koupal et al, and SAE Paper 900062, "Detection of Catalyst Performance Loss Using On-Board Diagnostics", William B. Clemmens et al. Since this system may be called upon to diagnose the effectiveness of the catalytic converter at any time, it must be capable of functioning over at least 100,000 miles of vehicle operation. Furthermore, since automotive engines may be utilizing a variety of hydrocarbon fuels and alcohol fuels such as, for example, gasolines, diesel fuels, natural gas, methanol, ethanol and mixtures thereof, both the engine control system, the catalytic converter and the diagnostic system for the catalytic converter must be capable of functioning in combination with such a variety of fuels.
Accordingly, it is an object of this invention to provide a sensing device capable of providing an electrical signal that is directly related to the amount of oxidizable constituents in the exhaust stream exiting the catalytic converter.
It is a further object of this invention to provide such a device that is capable of monitoring the oxidation of unburned hydrocarbons or carbon monoxide in the exhaust stream and producing an electrical signal proportional to the amount of such substances present. It is an object of this invention to provide such a device that is relatively small, quickly responsive to changes in the composition of the exhaust gas and yet durable so as to be operative over 100,000 miles of vehicle operation.
It is a more specific object of the invention to provide a calorimetric sensor having two members of identical thermal mass and exhaust flow profile, one catalyzed and one uncatalyzed, such that the catalyzed member promotes the exothermic oxidation of unoxidized constituents in the exhaust gas, thereby producing a related temperature increase of the catalyzed member with respect to the uncatalyzed member.
It is a further object of this invention to provide a method of utilizing such a two member sensing device so that the temperature difference in the device is only considered for diagnostic purposes during times of vehicle operation which most likely produce reliable data with respect to the efficiency of the catalytic converter.